Partial and complete disconnection of the human corpus callosum to treat intractable epilepsy has made possible the separate testing of each cerebral hemisphere without direct influence from the other. It has also made possible examining cortical versus subcortical processes active in interhemispheric integration of both simple and complex attentive, perceptual and cognitive processes. While the left hemisphere dominates for language and problem solving capacities, the minor right hemisphere is frequently superior at carrying out spatial, attentional, emotional and some perceptual processes. These data collected over the last 30 years have allowed for a theoretical characterization of human cortical function into sub-processes or modules. The modular concept argues there are many specific perceptual and cognitive capacities that have evolved to support human cognition and that these may correspond to definable anatomical entities. At the same time this framework has many important caveats, and intense experimentation is needed to resolve a variety of important issues. Among these issues are the following: 1) Assessment of the extent and variations seen in right hemisphere language by examining split-brain patients in an effort to clarify the respective roles of the two hemispheres in normal language processes (modular distribution). 2) Examination of the interaction and independence of the two disconnected hemispheres in perception and attention processes, with an emphasis on determining the exact capacity and extent of subcortical interactions that are possible following callosal disconnection (modular capacity). 3) Determination of the upper limits of perceptual capacities if the isolated right hemisphere, now made possible by our new ability to present prolonged lateralized stimuli using retinal-image stabilization; initial exploration of complex perceptual phenomena that may or may not require interhemispheric cortical communication (modular interaction). 4) Exploration of the emotional and memory functions of the two hemispheres via studies conducted in our new visiting scientist program. A variety of behavioral, psychophysical, and electrophysiological (ERP) methods will be employed to characterize the possible modular organization of human cognitive processes in patients who have undergone callosal surgery. Examination of both pre- and post- operative perceptual and cognitive capacities will also illuminate the role normal interhemispheric interactions might play in allowing for mental processes believed to be the specialized activities of one hemisphere.